1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices, and more particularly to a method of heat treating a III-V substrate such as a GaAs substrate to reduce the dislocation density near the III-V substrate surface.
2. Description of the Related Art
Recently, III-V compound semiconductor devices have been developed for high speed and low power dissipation devices. For example, a GaAs metal-semiconductor field-effect transistor (MESFET), a GaAs light emitting diode (LED), a GaAs semiconductor LASER and so forth have been put to practical use.
In manufacturing these devices, active elements are made by using a semiconductor substrate or an epitaxial layer which is deposited on the substrate by the method of metal organic chemical Vapour Deposition (MOCVD), molecular beam epitaxy (MBE) or liquid phase epitaxy (LPE). However, in all cases, there is dislocation in the substrates so that the threshold voltage standard deviation (.sigma. Vth) of the devices such as MESFET's are influenced and deteriorated.
Thus, the substrates having no dislocation or a little dislocation are expected. The threshold voltage standard deviation (.sigma. Vth) for MESFET's on a liquid encapsulated Czochralski (LEC) grown GaAs substrate, which is annealed in an N.sub.2 atmosphere is reported by Yasunobu Ishii, et al in IEEE Transactions on Electron Devices, Vol. ED-31, No. 6, June 1984, pages 800-804.
In the report, the dislocation density is investigated as etch pit density (hereinafter referred to as "EPD"). As shown in FIG. 1, the commercially available LEC-grown undoped semi-insulating GaAs substrates show a typical W-shaped distribution of EPD. The threshold voltage (hereinafter referred to as "Vth") of MESFET's fabricated on these substrates show an M-shaped distribution corresponding to the W-shaped EPD distribution.
In fabricating the MESFET's on commercially available LEC-grown undoped semi-insulating GaAs substrates as mentioned before, the GaAs substrates are annealed in an N.sub.2 atmosphere so that the dislocation density near the GaAs substrate surface cannot be decreased and the uniform Vth distribution cannot be obtained.
Further in fabricating GaAs LED's or GaAs semiconductor LASER's, a high pure epitaxial layer deposited on the substrate by the method of MOCVD, MBE or vapor phase epitaxy (VPE) using a halide is influenced by the dislocation in the substrate so that the semiconductor devices are short-lived or cannot be fabricated with high yield. Accordingly it is necessary to deposit the epitaxial layer on an expensive substrate with low dislocation density so that there is a problem that the semiconductor devices cost too much.